1. Field of the Invention
This invention relates to the production of elemental sulfur and more particularly to a new and improved process for the recovery of sulfur from sulfur dioxide.
2. Description of the Prior Art
Legislation enacted in certain states requires the removal of a considerable portion of the sulfur dioxide from the off gases of copper smelters, prior to discharge of the gases into the atmosphere. Sulfur dioxide is currently removed from the sulfur dioxide-containing off gases of copper smelters in accordance with one process by absorbing the SO.sub.2 gas in a liquid organic base such as an aromatic amine, e.g. N,N-dimethylaniline, by contacting the sulfur dioxide-containing off gases or off gas with the liquid N,N-dimethylaniline absorbent, and then stripping the SO.sub.2 gas from the pregnant absorbent. The thus released SO.sub.2 gas is converted into liquid SO.sub.2.
The conversion of the SO.sub.2 gas into sulfuric acid at the smelter tends to pose certain problems. Considerably more sulfuric acid can usually be produced in the smelter acid plants than can be disposed of in the marketing areas available to the smelters. It is not practical to ship sulfuric acid to the distant markets due to the appreciable expense of transporting the acid, and unlimited storage facilities for the acid is also impractical due to the appreciable expense, hazards posed, etc. Disposal of the excess sulfuric acid by dumping the acid onto the ground is ordinarily not a satisfactory solution as this means of disposal may pose pollution problems and also may pose safety problems due to the highly corrosive nature of the acid.
U.S. Pat. No. 1,880,741 discloses the reduction of sulfur dioxide to elemental sulfur by passing the sulfur dioxide and a hydrogen-containing gas over a partly reduced sulfide of iron, nickel or cobalt as catalyst at an elevated temperature of about 180.degree. C. to about 300.degree. C. The catalyst can be prepared by heating a sulfate of iron, nickel or cobalt, or any mixture of these compounds to a high temperature, preferably about 600.degree. C., then lowering the temperature and passing sulfur dioxide and hydrogen or sulfur dioxide followed by hydrogen over the catalyst. U.S. Pat. No. 3,149,920 relates to the production of sulfur from sulfur dioxide by adding sulfur dioxide to a gas stream containing hydrogen sulfide, and passing the resultant gaseous mixture over an initially anhydrous bed of catalyst at a temperature in the range from about 25.degree. C. to about 200.degree. C. in the presence of a hydrocarbon solvent. The catalyst can be silica, alumina, silica-alumina, silica-zirconia, silica-alumina-zirconia, silica-magnesia, silica-boria and silica-magnesia-alumina. U.S. Pat. No. 1,773,293 discloses the production of elemental sulfur by passing a gas mixture containing hydrogen sulfide, sulfur dioxide and ammonia vapor in contact with activated bauxite at temperatures below 200.degree. C. U.S. Pat. No. 3,454,355 discloses the removal of sulfur dioxide and nitrogen oxides from gaseous mixtures by contacting the gaseous mixture at a temperature of at least 750.degree. F. with cobalt, nickel, silver molybdenum, copper, palladium or alumina as catalyst, in the presence of carbon monoxide in an amount at least 0.75 of the stoichiometric amount necessary to reduce the sulfur dioxide and other oxidizing gases in the gaseous mixture. U.S. Pat. No. 2,361,825 relates to the reduction of sulfur dioxide to elemental sulfur and hydrogen sulfide by contacting the sulfur dioxide-containing gas at a temperature from 600.degree. F. to 2400.degree. F. with hydrogen and, as catalyst, a sulfide of iron, cobalt or nickel supported on aluminum oxide.
We found that the use of a supported oxygen-containing compound of a metal of the iron group pf Group VIII of the Periodic Table which was capable of undergoing a significant increase in its molecular volume during the contacting of the gas mixture comprising sulfur dioxide, carbon monoxide and hydrogen therewith, such as cobalt oxide, as catalyst in the first stage reaction zone was unsatisfactory due to decrepitation and crumbling of the supported catalyst in the initial upstream, high heat-release portion of the first stage reactor tubes. The high heat-release, which occured in the first approximately 10 to 12 inch length of the reactor tubes of the first stage reaction zone, was accompanied by the relatively low molecular volume cobalt oxide being converted to the relatively high molecular volume cobalt sulfate with attendant decrepitation and crumbling of the supported catalyst. The catalyst fines produced by the crumbling resulted in disadvantageous packing of the powdered catalyst or fines and blockage of gas flow therethrough.